1. Field of the Invention
This invention relates to the separation of oil-water mixtures. In one aspect, it relates to a method for use in offshore operations whereby oil and water mixtures produced from an offshore well can be separated to permit the discharge of water into the sea.
2. Description of the Prior Art
Most oil and gas wells produce a certain amount of water or brine in conjunction with oil and gas. A major problem faced in offshore oil operations is the economic separation of such oil, gas and water mixtures so that the water can be removed from the hydrocarbons and discharged back into the sea. The separated water returned to the sea must have a quality that complies with environmental discharge laws and regulations. For example, recent guidelines issued by the Federal Environmental Protection Agency requires that water discharged into the Gulf of Mexico may contain no more than 72 parts per million of oil. Conventional separation devices such as heater treaters normally discharge an aqueous effluent that typically contains 200 to 300 parts per million of oil and frequently as much as 1000 parts per million.
Numerous methods and apparatus have been employed by the offshore petroleum industry to reduce the oil concentration in produced water to environmentally acceptable limits. One approach has been to use fibrous media coalescers and collectors. Such devices utilize cords, mops or filter elements which contain oleophilic or petrophilic fibers. The primary function of these fibrous elements is to selectively coalesce droplets of oil while rejecting water so that the droplets can be more readily adsorbed and collected.
Fibrous media coalescers use a stationary oleophilic filter element to attract oil particles from the oil and water mixture. (See, for example, U.S. Pat. Nos. 3,668,118 and 3,794,583.) Although coalescers are effective in substantially reducing oil content in an effluent stream, they have a tendency to get saturated with oil or plugged with suspended solids. As a result, frequent cleansing or replacement of the filter element may be necessary. Suspended solids and fines are especially a problem since the build-up of solids on the fibers causes their performance to rapidly deteriorate.
One approach to solving the problem of oil saturation and solids build-up is to continually expose the oily effluent to relatively clean oleophilic fibers. A device which employs such a technique is the socalled "Oil Mop" (a tradename of Oil Mop, Inc.). The oil mop, which is disclosed in U.S. Pat. Nos. 3,748,682 and 3,774,685, employs an endless loop of an oleophilic, fibrous cord that is drawn through a pool of water having a floating layer of oil. As the cord pulls through the water it contacts the oil layer and sorbs it. The oil-laden cord is then brought to the surface where it is sent through a wringer or squeezed to remove the oil and suspended solids from the cord. The cleansed cord is returned to the water to collect more oil. The endless, oleophilic cord is effective in collecting oil but has the disadvantage of having a contact area limited by the size of the cord. Therefore, it can only be used effectively when the oil can be concentrated in a layer floating on top of the water. If the oil is finely dispersed throughout the oil in fine droplets, as is the case with most waste water streams from an offshore rig, then the oleophilic cord, as it is currently used, will be unable to achieve any further significant clarification of the water.